When should a fiber-type, self-locking nut not be used on an aircraft?

Fiber-type, self-locking nuts are specifically designed to provide a reliable locking mechanism that prevents loosening under vibration and dynamic loads. However, they are not suitable when the bolt is subject to rotation. This is because the internal fibers of the nut may wear out or deform due to the continuous movement and friction, compromising their locking ability over time.

Using a self-locking nut in this scenario could lead to a loss of clamping force and potentially allow the bolt to loosen, resulting in mechanical failure. Aircraft components often experience various forces during operation, and ensuring that fasteners remain secure is critical for safety and performance. When rotation is involved, it's essential to choose fasteners designed for such conditions, which can provide the necessary holding power without the risk of failure associated with fiber-type self-locking nuts.

In contrast, other loading conditions like tension or shear can sometimes allow the use of these nuts, provided that the overall design and operating conditions assure sufficient locking capability despite those stresses.